Aria labels 2.0

[eslteacher902010]

This PR has the same accessibility and formatting improvements from PR #910 into the 2.0 branch, so both versions of the site are in sync.

It includes:

Adding aria-label attributes for accessibility in MainNavLinks and JumpToLinks

Fixing minor JSX indentation for improved readability

(These changes match  [PR #910](https://github.com/processing/p5.js-website/pull/910).)

Let me know if anything needs adjusting!